Lesson 21 — Activity 2: Supplementary Angles
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Lesson 21 — Activity 2:
Supplementary Angles
Lesson 21 — Activity 2:
Supplementary Angles
In the last lesson, you learned that two angles are complementary if they add up to 90 degrees.
Two angles are supplementary when they add up to 180 degrees (also called a straight angle).
Example of a straight angle.
Take a look at the pictures below for examples of where you might
find supplementary angles in real life!
In
Figure (i), notice that the two streets meet at angles A° and B° at the
corners. Each of these angles is 90°, so the sum of these two angles is
180°.
In Figure (ii), you can see a tree that has two branches.
You can see the supplementary angles D°, which is the
angle
of a
branch on the tree from the ground, and U°, which is the
angle
of the branch to
the sky. These angles, which are 90° each, add up to 180°, thus forming supplementary
angles!
These two angles are supplementary because they add up to 180°, as follows:
40° + 140° = 180°
Just like complementary angles, supplementary angles also do not have to be together (adjacent). Take a look at the example to your right.
The first angle is 60°, and the second angle is 120°, which add up to 180° so these are supplementary angles!
Here's a tip!
How can you remember which angle is complementary and which angle is supplementary? It's easy!
It's easy to find the missing angle in a set of supplementary angles. If one of the angles is known, its supplementary angle can be found by subtracting the measure of its angle from 180°.
For example, find the supplementary angle below (angle x).
You know that one of the angles is 130°, so to find the other angle, just subtract as follows:
180° – 130° = 50°
Therefore, angle x = 50°
Let's try another one where there are more than two angles on a line. In the figure to your right, you know three of the angles. The question is asking you to find out what angle x is. To do this, all you need to do is add up the three angles
and then subtract that sum from 180° as follows:
20° + 60° + 60° = 140°
180° – 140° = 40°
Therefore, x = 40°!
Click here to watch a video on how to find supplementary
angles.
Self-check!
Try This!
Find the supplementary angles below.
When you are done, click on the tab to check your answers!
